We also install waterproof membranes (often Polyglass' Duflex, a modified bitumen). Often we heat-fuse their all-black asphaltic system over a roof. Then, sometimes homeowners choose to install a "traffic surface" (concrete, tile, carpet, a wood deck, etc.) atop it, in a non-penetrating manner, to use the flat roof as a deck.

Here is an example of a modified membrane installation. Though it's not much to look at now, properly installed, this product is a fantastic roofing/waterproofing product.